The present invention relates to a screw for a thermoplastic extruder.
In the manufacture of thermoplastic material, the extruder receives the thermoplastic material in the form of pellets or chips in its hopper and delivers such material to the screw for processing. The screw has a feed section for advancing the solid or unmelted material at a predetermined rate, a melting section for plasticizing or working the material to turn it from a solid to a melted state, and a metering section for advancing the melted material to the discharge end of the extruder.
The conventional screw comprises a root and a single flight helically extending about the root. The flight forms with the interior cylindrical surface of the extruder barrel, a helical channel along which the thermoplastic material is conveyed. As the solid thermoplastic material enters the melting section from the feed section, the material begins to melt due to the heat created by friction within the thermoplastic material and heat from an external source conducted through the barrel itself. The melting rate depends a great deal upon the amount of contact between the barrel and the thermoplastic material in the solid state. The material in contact with the barrel begins to melt and form a melt film which adheres to the inner surface of the barrel. When the thickness of the melt film exceeds the clearance between the barrel and flight, the leading edge of the flight scrapes the melt film off the inner surface of the barrel and collects it at the forward or leading edge of the flight, forming a melt pool of the melted material. As the material continues to melt, the solid mass, normally referred to as the solids bed, eventually breaks up into clusters of floating solids in a stream of molten material. When this happens, only a small portion or none of the solid material is exposed to the barrel and heat must travel from the barrel through the molten material to the solid material. Since the thermal conductivity of thermoplastic material is very low, this is a very inefficient heat transfer condition. As a result, the melting efficiency is greatly diminished once the solids bed has broken up.
There have been many attempts to solve this problem by designing screws so that the capacity of the channel decreases toward the outlet end in an attempt to keep the solids bed intact as long as possible. However, most screw designs were unsuccessful in this attempt until the development of a screw having a primary flight and a barrier flight that defines a solids channel extending between the trailing edge of the primary flight and the barrier flight, and a melt channel extending between the leading edge of the primary flight and the barrier flight. The barrier flight has a smaller outer diameter than the primary flight. As the material in the solids channel melts, it passes over the barrier flight and is scraped off and into the melt channel, so that the solids and the melt remains separated as the thermoplastic material is conveyed along the length of the screw. However, since the ratio of the solids-to-melt gradually decreases as the material is advanced toward the exit end of the extruder, it is necessary to increase the volume of the melt channel while decreasing the volume of the solids channel.
U.S. Pat. No. 3,375,549 to Geyer issued Apr. 2, 1968 shows a barrier or double channeled screw in which the volume of the solids channel is diminished by decreasing its width and depth and the volume of the melt channel is increased by increasing its width and depth. This provides maximum exposure of the solids to the barrel at the inlet end of the screw and minimum exposure at the outlet end of the screw. The advantage of this screw is that it provides maximum exposure of the solids when it is most needed, that is, at the beginning of the melting process when there are more solids and the solids are relatively cooler. However, since the width of the solids channel diminishes to zero toward the outlet end of the screw, melting efficiency also diminishes.
U.S. Pat. No. 3,698,541 to Barr issued Oct. 17, 1972 represents an attempt to overcome the problem of diminishing melting efficiency in a "barrier" or double channeled screw. As disclosed in this patent, the width of the solids channel is initially greater than the width of the melt channel and the width is kept constant along the entire melting section by maintaining the pitch of the primary flight and barrier flight constant. Barr accommodates the changing solids-to-melt ratio by gradually decreasing the depth of the solids channel and increasing the depth of the melt channel. This relationship provides a constant area of contact between the solids and the barrel from one end of the barrel to the other. The principal disadvantage of screws of the type shown in the Barr patent is that in order to accommodate the volume of melt, as the material progresses down the screw, the melt channel has to become very deep. Since the melt channel is initially narrow, it approaches a square cross-section towards the end of the melting section. This creates a condition of poor circulation of the melt within the melt channel, resulting in possible degradation of, and thermally unstable, material. These and other difficulties experienced with the prior art screws have been obviated by the present invention.
It is, therefore, a principal object of the present invention to provide a screw of the barrier type that has maximum melting efficiency along the entire melting section of the screw.
Another object of the invention is the provision of a screw in which there is maximum solids-to-barrel contact along the entire length of the melting section of the screw.
A further object of the present invention is the provision of a screw in which the solids channel gradually changes in both width and depth while maintaining ideal width-to-depth ratios and without sacrificing melting efficiency.
It is another object of the instant invention to provide a screw in which the widths of the solids and melt channels gradually change, but at rates that vary in accordance with particular melt conditions existing in various zones in the melting section to maximize the melting efficiency of the entire melting section.